The country's Agriculture Ministry said that the H5N8 strain of bird flu had been detected on six poultry farms and that there had been 13 cases in migratory birds since the first outbreak earlier this month.

No human infection has been reported, while the ministry is looking into four additional reports from poultry farms and more than 50 other suspected cases in migratory birds, it said in a statement on Monday.

Asia's fourth-largest economy has had four bird flu outbreaks in the past 10 years, without any cases of human infection reported.

South Korea will slaughter over 1.4 million farm birds, including 644,000 that have already been killed, according to the ministry. That would be under 1 percent of the country's total 160 million poultry population.

The first case of H5N8 bird flu was found on January 17 at a duck farm in the southwestern province of North Jeolla, about 300 km (186 miles) from Seoul.

The government is stepping up disinfection measures for migratory bird at 37 sites across the country as it suspects they are the source of the latest outbreak.

It will also control the movement of some livestock workers in affected areas ahead of the Luna New Year holidays from Thursday through Sunday when many Koreans travel to family gatherings.